Atherosclerosis (or arteriosclerotic vascular disease) is a condition in which an artery wall thickens as a result of deposition of materials such as cholesterol, fat, and other substances resulting in the formation of hard structures called plaques. Formation of plaques makes the arteries stiff and narrow (stenosis), thereby restricting blood supply to the involved organs. Such restricted blood supply would damage the organ, and eventually lead to its failure. Pieces of plaque can also break away and move from the affected artery to smaller blood vessels, block these vessels completely, and consequently, result in tissue damage and death (embolization). This embolization process is one of the causes of heart attack and stroke. Recent statistics from the World Health Organization indicate that such cardiovascular diseases (CVDs) are the world's largest killers, resulting in 17.1 million deaths a year. Estimates indicate that by 2030, almost 23.6 million people will die from CVDs, mainly from heart disease and stroke. The earliest risk indicator of a possible CVD is the presence of atherosclerosis. Early detection of atherosclerosis and adequate treatment and lifestyle changes would enable the patient to prevent the onset of a CVD. Unfortunately, atherosclerosis is a chronic disease that remains asymptomatic for decades. Therefore, development of techniques that detect the presence of plaque at its earliest stages is of paramount importance.
Owing to its anatomical position and its relatively large diameter, the Common Carotid Artery (CCA) is the preferred artery for routine clinical examination to detect the presence of plaque and to study its characteristics. The examination is mostly carried out using non-invasive carotid artery ultrasound, which is a cost-effective and relatively fast imaging technique that does not use ionising radiation. However, ultrasound technique is operator dependent, and hence, the interpretation is subjective. Moreover, even though studies show that ultrasonographic B-mode characterization of plaque morphology is useful in assessing the vulnerability of atherosclerotic lesions, a confident and reproducible classification of dangerous plaques and its risk score from this plaque is still not available. Also, the correlation between ultrasonographic findings and the histological findings of carotid plaques is often poor. These limitations are due to the low image resolution and artifacts associated with ultrasound imaging. Thus, Computer Aided Diagnostic (CAD) techniques that adequately pre-process the ultrasound images before extracting discriminating features for further classification and giving a risk score would improve the correlation between ultrasound based results and histological results of carotid plaques.
Once plaque is detected, the treatment options to unblock the stenosis include Carotid Artery Stenting (CAS) and Carotid Endarterectomy (CEA). CAS is a non-invasive procedure that uses a catheter to correct stenosis, whereas CEA is a surgical procedure. It has been shown that CEA reduces the risk of ipsilateral stroke. In a clinical trial named CREST, both CAS and CEA were compared on the collective incidence of any stroke, any heart attack or death. The study concluded that there was a higher risk of stroke with CAS and a higher risk of myocardial infarction with CEA. This conclusion indicates that there is a considerable risk for the patient undergoing either of these procedures. Therefore, characterization of the patient as symptomatic or asymptomatic based on the wall plaque morphology would enable the vascular surgeons to decide whether the patient needs such risky procedures. This is because studies have shown that only symptomatic patients were found to have more frequent plaque rupture that cause life-threatening embolization. Plaque rupture was seen in 74% of symptomatic plaques and in only 32% of plaques from asymptomatic patients.
Thus, the development of an efficient completely automated CAD technique that not only detects plaque but also classifies it into symptomatic and asymptomatic while giving a cardiovascular risk score would immensely assist the doctors in managing atherosclerosis.